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Octane Ratings Explained

Exhaust smoke

Most people are aware that octane ratings affect an engine's performance in both power output and fuel economy, but how this works and whether it's relevant to your vehicle in particular is not always as well understood. To understand what octane means to a typical four-stroke gasoline engine, we need to understand how that engine works.

As mentioned, there are four strokes to the combustion process in a typical automotive gasoline engine. The intake stroke, the compression stroke, the power stroke, and the exhaust stroke. Each stroke is a full movement of the piston inside the cylinder downwards or upwards.

During the intake stroke, air and fuel are injected into the cylinder as the piston moves downward, creating a vacuum for the air:fuel mix to fill. During the compression stroke, that air:fuel mix is pushed into a smaller and smaller area as the piston moves back upwards. Just before the piston reaches its top-most point, spark ignites the air:fuel mix and starts the power stroke. The sparked fuel combusts in the air (oxygen) and decompresses, forcing the piston back down. The piston then returns and exhaust valves open to release the exhaust gases left over from the completed combustion.

Octane plays several roles in the four-stroke process. The octane rating is not a level of oxygen in the fuel, but is instead a number representing the resistance the fuel has to ignition. This number correlates to how much compression can be given to the fuel when mixed with air before it spontaneously ignites from the heat created by the pressure.

Most critical is that combustion point and how resistant the fuel is to premature combustion in the chamber. Higher octane fuels allow for higher pressure (compression) which means more power is produced from less fuel and air. There is a point, however, where this is moot.

In many engines, especially economy-minded low-compression engines such as those found in many hybrids, higher octane makes little different to the car's overall fuel economy. Because there is little chance of lower octane fuels combusting without spark, there isn't much to gain from the higher octane fuel. On the other hand, some vehicles require higher octane fuels because of their higher compression ratios or specific engineering.

Other factors, such as altitude (which affects the oxygen content of the air), can also play a role in how much octane is required.